In injection molding of thermoplastic material, such material is fed in molten form from the extruder barrel of an injection molding machine through an injection nozzle, which is adapted to engage an injection bushing through which the material is supplied to the interior of the mold in which the material cools and solidifies to form the desired object. The quality of the material of the molded object is dependent to a considerable degree upon the extent to which the material is uniformly maintained at an optimum processing temperature throughout its residence time in the extruder barrel and injection nozzle. If the temperature of the injection nozzle is permitted to drop below the temperature of the material in the extruder barrel, the material delivered to the injection bushing may be too cold for optimum results, while if an attempt is made to heat the injection nozzle by separate heating means, the material may become too hot for such optimum results. Also, additional heaters are expensive and introduce system control problems.